This invention relates generally to ball valves. More particularly, this invention relates to thermoplastic ball valves of the end entry type.
End entry type ball valves have well recognized advantages from the standpoint of manufacturing and assembly. In known thermoplastic end entry ball valves, such as the type shown in U.S. Pat. No. 3,550,902, additional advantages are offered by the provision for adjusting the parts of the assembly when wear occurs upon the seals.
While compensation for wear is desirable, control over the contact relationship between the seal member and ball is also an important factor. This relationship can be affected by operating conditions as well as by assembly operations.
For example, external forces generated by thermal expansion or contraction, piping misalignment, and initial positioning of the seal member can all, in the absence of precision tolerances, materially affect the sealing relationship. With thermally induced expansion, the seal members can be pushed very tightly against the valve ball, and perhaps result in undesirably high operating forces and/or marring of the ball. Thermally induced contraction can result in loss of the seal. Excessive tightening of the seal member against the ball during assembly or misalignment of the desired assembled relationship of the seal member and ball can lead to permanent deformation of the seal member itself and other undesirable problems.